The present invention relates to a control system for automobile engines, or more in particular to a gasoline engine control system using learning control for determining the fuel flow rate in the feed-back control of air-fuel ratio.
Automotive gasoline engines developed recently are equipped with a control system including a microcomputer for controlling the operating conditions thereby to reduce the harmful components in the exhaust gas and improve fuel economy. In fact, an electronic engine control system operates in such a way that in response to signals from various sensors representing engine operating conditions, various factors including fuel supply rate and ignition timing are controlled thereby to attain the optimum engine operating conditions.
An example of an electronic engine control system is disclosed in JP-A-55-134721 filed for Japanese patent by Hitachi Ltd on Apr. 6, 1979 and its corresponding U.S. Pat. No. 4,363,097 issued on Dec. 7, 1982.
In the typical electronic engine control system of this type, the fuel flow rate is controlled by an air-fuel feedback method. According to such a method, the intake air flow detected by an air flow sensor and the engine speed are used to determine a basic fuel injection rate. This basic fuel injection rate is multiplied by an air-fuel ratio feedback factor making up a feedback value corresponding to the oxygen concentration in the exhaust gas and other compensation factors representing control parameters. A battery voltage compensation is added to the resulting compensated basic fuel injection rate thereby to determine a required fuel injection rate. By a drive signal corresponding to the required fuel injection rate thus calculated, the opening time of an injector nozzle is controlled to maintain the air-fuel mixture at a target air-fuel ratio (stoichiometric air-fuel ratio).
The required fuel injection rate is thus represented by the pulse width of the drive signal applied to the injector The pulse width T.sub.P of the drive signal corresponding to the basic fuel injection rate that is the basic fuel injection pulse width and the pulse width Ti corresponding to the required fuel injection rate (hereinafter referred to as "the required injection pulse width") are given as EQU T.sub.P =k.times.Q.sub.a /N ... (1) EQU Ti=T.sub.P .times.K.times..alpha..times.K.sub.L +T.sub.S...(2)
where k is a constant, Q.sub.a an intake air amount, N an engine speed, K a compensation factor due to the engine cooling water temperature, etc., .alpha. a compensation factor for the air-fuel ratio feedback, K.sub.L a learning compensation value of fuel flow rate, and T.sub.S an ineffective pulse width of injector (battery voltage compensation).
Specifically, by use of the intake air flow rate Q.sub.A of the engine and the engine speed N, the basic fuel injection time T.sub.P is determined from equation (1). The resulting value of the basic fuel injection time T.sub.P is multiplied by the air-fuel ratio feedback compensation factor .alpha. thereby to determine a fuel injection flow rate associated with a target air-fuel ratio (stoichiometric air-fuel ratio). In the fuel flow rate control system of an actual engine, the input-output characteristics of the various actuators (such as a fuel injector) and sensors (such as an air flow sensor) are subjected to secular and other variations. To control the fuel flow rate only with the feedback compensation factor o is not sufficient but secular and other variations are required to be compensated by learning thereby to secure accurate air-fuel ratio control. The compensation thus achieved by learning is provided as a learning compensation value K.sub.L.
The learning compensation value K.sub.L will be explained in more detail. An O.sub.2 sensor disposed in the exhaust pipe produces a binary signal (high-level voltage for rich and low-level voltage for lean mixture) in accordance with the oxygen concentration (lean for high and rich for low oxygen concentration) in the exhaust gas. This binary signal is used to increase or decrease the air-fuel ratio feedback factor .alpha. stepwise, followed by gradual increase or decrease respectively to approach a target air-fuel ratio. FIG. 1 shows the conditions of the air-fuel ratio feedback factor .alpha. which undergoes a change upon detection of a rich- or lean-side value of the air-fuel ratio in response to the output signal .lambda. of the oxygen sensor.
With regard to the air-fuel ratio feedback factor .alpha. with the oxygen sensor signal reversed in direction, the local maximum value in the process of change from lean to rich state is assumed to be .alpha..sub.max, and that in the process of change from rich to lean state to be .alpha..sub.min. The average value of the two .alpha..sub.ave is given as EQU .alpha..sub.ave =(.alpha..sub.max +.alpha..sub.min)/2... (3)
The deviation between the average value .alpha..sub.ave shown in equation (3) and unity is defined as a learning compensation value K.sub.L. In other words, EQU K.sub.L =.alpha..sub.ave -1... (4)
If the air-fuel ratio feedback factor .alpha. is 1, it is the same as if the air-fuel ratio has attained a target value without air-fuel feedback control by the oxygen sensor.
The learning compensation value K.sub.L varies from one engine operating region to another, and therefore a memory has stored therein a learning compensation value for writing the learning compensation value K.sub.L for each operating region indicated by the engine speed and the basic fuel injection rate (pulse width) shown in FIG. 2. In an operating region subjected to air-fuel ratio feedback control, the calculation of equation (4) is performed with the K.sub.L value written for the particular region on the map. Further, in order to compensate for the secular variations of the fuel flow rate control system, each value of K.sub.L in the map is learned and updated during the operating period. At the time of calculation of the required fuel injection rate, the memory is read to use the learning compensation value K.sub.L of an associated operating region. The learning compensation value K.sub.L is learned, that is, updated at a time when the engine operating region remains unchanged while a predetermined number of local maximum values of the air-fuel ratio compensation factor .alpha. occur in succession, that is, when the operating condition is not transient. The map of FIG. 2 is divided into a total of 64 operating regions. In general operation of a car, it is a rare thing to use all the regions on the map. The learning compensation value K.sub.L for an unlearned or unexperienced operating region is calculated by estimation from the learning compensation values K.sub.L for the regions surrounding the particular operating region.
The learning control of the air-fuel ratio is disclosed, for example, in JP-A-60-65254 filed for patent in Japan by Hitachi Ltd. on Sept. 20, 1983 and JP-A-60-111034 field for patent in Japan by Hitachi Ltd. on Nov. 21, 1983 and the U.S. Pat. No. 4,703,430 issued on Oct. 27, 1987, and claiming the priority right based thereon.
The conventional learning control systems are for compensating for variations in input/output characteristics or secular variations of all sensors and actuators of a fuel injection control system only by means of a single learning compensation value K.sub.L. Further, as seen from equation (1), the basic fuel injection rate T.sub.P is not compensated at all. The air flow rate sensor of an air flow sensor such as a hot-wire sensor for detecting the intake air flow Q.sub.a, however, has an input/output characteristic thereof which is sometimes subjected to variations in the course of production or initial input/output characteristics thereof sometimes undergoing variations due to dust or oily contamination attached thereto in the course of operation. As a result, the value of the intake air flow Q.sub.a detected has some error. If the intake air flow Q.sub.a has an error, the basic fuel injection rate T.sub.P naturally develops an error. The basic fuel injection rate T.sub.P corresponds to an engine load, on the basis of which the optimum ignition timing is determined, and therefore an error in the intake air amount of the air flow meter would lead to an improper ignition timing, resulting in a reduction in engine performance or fuel efficiency or a case of knocking.
Changes in input/output characteristics due to secular variations, on the other hand, are not limited to the air flow meter, but occur also in the fuel injector. The injector nozzle, in particular, is liable to be reduced in diameter by dust in the fuel or carbon due to backfire deposited thereon to reduce the fuel flow rate. The secular variations of the fuel injector thus cause an error in fuel injection rate, thereby greatly affecting the control of the air-fuel ratio.
In view of these facts, if an accurate air-fuel ratio and an accurate ignition timing are to be attained in the learning control of the fuel injection rate, it is necessary to compensate separately for the variations in input/output characters of the air flow meter and the fuel injector.